Cumulonimbus clouds do not form in tornadoes. Tornadoes are violent rotating columns of air that extend from the base of a thunderstorm cloud to the ground. The vast majority of tornadoes occur from Cumulus congestus clouds which, if they continue to grow, will transform into a supercell thunderstorm if the atmospheric conditions are right. Cumulonimbus clouds are often associated with severe thunderstorms, but they do not directly cause tornadoes.